1. Field of the Art
The present invention relates to a method of removing efficiently and economically an ammonium compound contained in coal ashes captured by a dust collector in a system which comprises steps of denitrification (hereinafter referred to as NO.sub.x removal) of an exhaust or flue gas containing coal ashes from a coal-burning boiler, cooling of the gas and dust collection on the gas.
Exhaust gases from coal-burning boiler contain coal ashes or flue dust, an oxide of nitrogen which is called "NO.sub.x " hereinafter, and an oxide of sulfur which is called "SO.sub.x " hereinafter, and should thus be subjected to dust removal, NO.sub.x removal and SO.sub.x removal treatments. The gases are suitably cooled before being subjected to the treatment or treatments usually by means of heat exchange of the gas with air required for burning coal. Usually, dust removal is conducted by means of, for example, an electric precipitator and NO.sub.x removal is conducted by catalytic reduction of NO.sub.x with ammonia. It is our position that since it is preferable to operate electric precipitator at a relatively low temperature it is preferable to conduct these treatments in the sequence of NO.sub.x removal, cooling, and dust removal. NO.sub.x removal is thus conducted on the exhaust gas still containing coal ashes in high concentration and the dust removal is thus conducted at a lower temperature on the gas which has undergone the NO.sub.x removal by the catalytic reduction of NO.sub.x contained therein with ammonia usually used in a stoichiometrically excess quantity.
In the system which is specifically designed for the steps of the NO.sub.x removal from an exhaust gas containing coal ashes in a high concentration from a coal-burning boiler by introducing it into an NO.sub.x removal unit of the type for catalytic reduction with ammonia, cooling the gas by passing it through a heat exchanger, removing the coal ashes from the gas by passing it through a dust collector such as a low-temperature electrostatic dust precipitator, and thereafter subjecting the gas to SO.sub.x removal by introducing it into a desulfurization unit, a substantial quantity of ammonium compounds such as ammonium sulfate is produced through the reaction of the unreacted or leak ammonia from the NO.sub.x removal unit with the SO.sub.x contained in the exhaust gas and is then put into the coal ashes captured by the dust collector. It is often afraid that such ammonium compounds may very possibly give adverse effects in many ways not only in the handling at the coal ash processing facilities, but also in the disposal or in the recycle use of such coal ashes. The reduction in quantity of such unreacted ammonia from the NO.sub.x removal unit has been attempted somehow on the one hand but under existing circumstances with only unfavorable results attained in the meantime, and essential is thus a prompt countermeasure with respect to the handling of such ammonium compounds on the other.
It is noted that the questioned ammonium compound is typically found in the form of (NH.sub.4).sub.2 SO.sub.4 and/or NH.sub.4 HSO.sub.4 in such coal ashes. Typical methods of removing such forms of ammonium compounds from coal ashes are rinsing or leaching with water and thermal decomposition. With the water-rinsing method, while a high efficiency of removal of such ammonium compounds may be expected for a form of ammonium compound such as ammonium sulfate which is highly water soluble, there is still left the question of restricted use of coal ashes due to its high water content because of the rinsing with water. Moreover, there is another problem such that required is an additional treatment of a large quantity of the waste water produced in the rinsing process which contains substantial quantity of the ammonium compounds leached from the ashes.
The thermal decomposition, on the other hand, has some drawbacks, and is thus not satisfactory, either. The thermal decomposition of the ammonium compounds as mentioned above occurs in the manner represented in the following chemical equations; i.e., EQU (NH.sub.4).sub.2 SO.sub.4 .fwdarw.NH.sub.3 +NH.sub.4 HSO.sub.4 EQU NH.sub.4 HSO.sub.4 .fwdarw.NH.sub.3 +SO.sub.3 +H.sub.2 O
The thermal decomposition reaction of the ammonium compounds requires that it be conducted at quite a high level of temperature, and consequently, the provision of a heat source has been an essential question to meet in practice. Another possible problem is concerned with disposal of the gas produced from the thermal decomposition step. The gas contains ammonia, and it may thus be easily thought of to recycle the gas to the NO.sub.x removal step thereby to utilize the ammonia as the reducing agent. In view, however, of the fact that the gas can also contain an SO.sub.x such as SO.sub.3, the recycle of the gas to the NO.sub.x removal step would result in just a formation of ammonium compounds from the NH.sub.3 and the SO.sub.x instead of the use as the reducing agent of the ammonia.